1997 4.0L runs rich


seva.zaikov

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Hi everybody,

I recently bought my first truck, and it is a pretty old Ranger, 1997 2wd manual 5spd 4.0L engine with 155k miles on it. The truck runs strong, transmission works nice, there seems to be no leaks, and the price was good. But it runs rich and gets about ~12mpg on the highway, ~10 in the city.

Now, to the issue. There are bunch of codes regarding the O2 sensors: P1152, P0155, P1132, P0135, P0141. I know that they are known to fail (many I've looked had similar problems), so I bought 2 upstream NGK O2 sensors. They were stuck, so I got it to the shop, the installed them, and also replaced a fuel filter.

However, after resetting all codes they came back ~10 miles later (all of them).

Another issue is that engine temperature gauge does not work, but I checked hoses and it felt fine (so no overheating). The shop was not able to figure it out, they said the sensor was alright, and the harness was also working properly. I got an OBDII scanner, and got the following information:

RPM during idle was ~1400 when cold, got down to ~900 after warming up (seems normal)
Coolant engine temperature went up only up to ~153. I read that the operating temperature should be around ~190, so does it mean that thermostat is stuck open, or for lower temperature?
Fuel system status said: open loop due to detected system fault. I've researched and it seems that on some vehicles, especially old, it can be stuck open by PCM if you have any error codes.

Things I will be doing this weekend:

- clean MAF
- replace ECT sensor (I have the part already, and will test the temperature after)
- replace spark plugs/wires (just as a part of the tuneup, I don't expect it to fix anything)

Should I replace the thermostat as well, if another sensor shows the same?
What else can be a problem?

Also, there are no misfiring, or rough idling, any hesitation, or lack of power. Only running rich.

Thanks!
 


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alwaysFlOoReD

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Thermostat is cheap - replace it. I suspect that is the problem. If the temp stays low then the engine computer is treating it like a cold engine and so extra gas.
What about the other two o2 sensors?
 

seva.zaikov

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Yeah, I think I will do it, thanks – I think nothing except thermostat affects the temperature, so it definitely should at least bring it up to ~190.
However, after I've read that the loop is stuck open due to error codes, I am not so sure whether it might solve it.

In my model there are only 3 O2 sensors – 2 upstream, 1 downstream. Originally, I decided to not to replace just because I was under impression that it does not affect anything, except emissions report, so I thought about replacing it later. Now I am not so sure (again, about open loop), and also, was I right that the one after a cat converter does not affect the air/fuel mixture ratio?
 
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alwaysFlOoReD

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You are right, the one after the cat is a check to make sure the first 2 are working correctly. I don't believe it will have anything to do with fuel. However the thermostat if stuck open, will definitely have an effect on the computer, as in making it run open loop.
 

seva.zaikov

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Alright, so I finally worked on the truck today.

Things I've done:

- cleaned the MAF sensor (looked fine actually)
- replaced the third O2 sensor, which is after the cat converter (mostly for the piece of mind)
- replaced spark plugs & wires (incredibly tight spaces!)
- replaced a thermostat
- replaced ECT sensor (not the sender to the gauge)

Good news is that indeed there was a problem with a thermostat. Now, not only the scanner shows that the temp goes up to 190, but the gauge actually works! It is kind of lower than the center, but it actually moves.

The bad news is that it seems all trouble codes are still here. I idled it for about ~10 minutes to warm it up, and my scanner tells me that there are same pending trouble codes. I don't know about MPG yet, but seems it still runs rich.

What else can be the reason? Is there a way to test MAF sensor (I am not very keen to replace it blindly). Can it be a vacuum leak (shouldn't it run lean in this case?). Can it be fuel injectors related? I was thinking about adding seafoam to the fuel, so it can clean them. I've never done it before, so not sure about it.
 

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3 of those codes are O2 sensor heater circuit codes. That's where I would start.
The O2 sensors are 4 wire sensors, 2 of the wires are for the "pre-heater" (the 2 on each sensor with the same color wires). RonD will be along sooner or later with wiring diagrams, videos, explanations, (27 8x10 color glossy photos with circles and arrows and a paragraph on the back explaining what each one was). For now, just know that there should be power and ground on either of those 2 wires, when you turn the key on... see which one you don't have.
 

seva.zaikov

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Thanks for the suggestion, sounds reasonable. I've never tested the wiring before, so I might ask some dumb questions.

First of all, do I check only truck's connectors, or sensor itself, or both?

Another question is how to test.
If I understand correctly, to test the ground, I need to measure resistance, and put one piece to the wire, and another to the metal frame, and should see something close to 0. Is that correct?
To test the voltage, I should measure voltage on a multimeter, and put one piece to the wire, and another to the ground (so, either another wire which I proved working, or a metal frame). Is it correct?

Last part is about voltage number. Which number should I see? Where to get this info?
And what if I find that there is no power, or the ground has some resistance. What do I do in that case?

Thanks a lot, I appreciate your answers!
 

seva.zaikov

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Hi,

I checked the voltage using the link above, by reading it from OBDII codes. I looked at all of them (all 3), and they actually all show steady 0 (not a single fluctuation). Assuming this data is correct, and the fact that I replaced O2 sensors recently, I guess the culprit is somewhere here. They all are powered through a single fuse, so this might be a problem, but I looked into it, and all fuses look good. I also pulled out the fuse and checked for continuity, and it was good. I replaced it with a spare one, just in case, but nothing changed.

What should be my next step? Which connections to check?
 

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With the key on check for power at that fuse. It's a quick, easy place to test for power, and it services all three legs of the circuit, so it is a good place to start looking for a systemic problem.

Remember to test both poles of the fuse for power. Good resistance is not the same as good power carrying ability.
 

seva.zaikov

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Just checked all the fuses under the hood (including the one which services all O2 sensors). All of them were good.

The way I tested them was to put 20V mode in my multimeter, put the negative one to the ground (I used the frame and negative side of the battery) and connect to the fuse with the other side. It showed 12V, which I assumed means that is powered.

Is it the correct procedure? If so, what should be the next step? I never worked on the wiring before, so don't even have an idea what to check =\
 

seva.zaikov

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Giving it more thought, the main issue is that all of them throw heater circuit malfunction. Since I replaced all the sensors, I don't blame them. If I understand wiring correctly, it goes from the PCM to the fuse and then to the sensors.

Sensors are good, and the same for the fuse, so it leaves me with two options:

- problem in the wiring
- problem in the PCM

I am a little bit confused how to test it, though. First, I'll need to test actual O2 wiring connector to see whether there is power to the heater, proper ground, and same for the signal wires (although, I am not sure whether signal wire should have any voltage).

But how to analyze this info?

Let's say that I have no power in the sensors. I guess next step would be to test voltage in corresponding wires which are coming from PCM. How do I do that?
Another possibility is that I have power. Will it mean that the signal wire is broken? How to test this wire?

Which test will tell me that the PCM is to blame? Do I need to bring it somewhere to verify that the PCM is the problem (I read somewhere they have a special machine which loads it with different scenarios and they can confirm it is an issue)? How much will it cost to test it? If the PCM is the issue, can I just buy a new one (https://carcomputerexchange.com/index.php?act=viewCat&make=Ford&model=Ranger&year=1997&cSearch=FIND+COMPUTER) and replace it? Will my keys continue to work?

Thanks!
 

seva.zaikov

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Finally tested the actual O2 sensor connector. I used the downstream sensor (others were too much hassle to reach with a multimeter).

Voltage was fluctuating, max I saw was 8V, average was around 2V. I also tried to test the continuity by measuring resistance, but no luck. So I wanted to find the ground, but it turned out to be extremely hard. There are some wires which are bolted to the frame here and there, but they all seem good. How to find where O2 sensors are grounded? The engine compartment is so cramped that I can't just follow the wires, unfortunately.

p.s. I also checked all the fuses under the hood, they are all good (I put the negative piece to the - side of the battery and measured voltage on both sides of a fuse).
 

seva.zaikov

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I finally managed to go to the dealership with the truck. They thought originally it is bad wiring somewhere, but later they tested the PCM and it was the culprit. So, I got unlucky :) Now it runs smooth with proper mpg.
Thanks for all the advice!
 

alwaysFlOoReD

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Glad you got it fixed. Thanks for the update.
 


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